It is, but (no offense to Slow) the arrangement didn't come out very well. The second half is in the wrong key! (that might not sound too important, but if you have perfect pitch, it makes you go AAAAAUUUUUUGH!!)

I think it'd be better if you used grace notes instead of the 32nd notes.those 16th triplets could be replaced by a mordent.

Bowser's Theme:

feels a bit too slow. I think the tempo is actually 115

Dark Cave (Duet)*:

try to keep the parts balanced. you should give piano 2 the melody as well.

Secret Island*:

I'd replace those tied eighth notes with quarters except in between beat 2 and 3.in the left hand, I'd make the rhythms staccato quarter note, eighth rest, eighth note tied to a quarter, quarter.47-50 - is it necessary to have those notes in the bass that low?

if it's a one piano duet, why are there two grand staves with treble and bass clefs in each of them? if you wanted to make a proper "four hand" duet, you'd have piano 1 renamed "primo" and piano 2 renamed "secondo". also primo should have both hands in treble clef and secondo should have both hands in bass clef(with exceptions).also even if it was for one piano, the melody and that accompany part would clash.